To cope with the recent reality of stringent shortage in frequency spectrum due to the proliferation of wireless services, cognitive radio has been considered as an attractive technique to improve spectrum utilization for future wireless systems. In cognitive radio networks, one important function of secondary transceivers is to determine when primary transceivers are utilizing a channel, and to access the channel in such a way that it causes little performance degradation to the primary transceivers. Previous attempts at detecting the usage of a channel by one or more primary transceivers have had limited performance, especially at low signal-to-noise ratios.